1. Field of the Invention
The present invention relates to a thermal recording material, and more specifically to a thermal recording material which, by using a protecting layer made from a coating solution not having a crosslinking agent combined therewith, can contribute greatly to improvement in productivity, and solves various problems caused by the crosslinking agent, and at the same time, has improved heat resistance, water resistance and chemical resistance.
2. Description of the Related Art
Generally, a recording material which comprises a recording layer formed on a support and mainly composed of an electron donating colorless or lightly colored basic dye and an electron accepting organic or inorganic substance and having a binder, a filler, a sensitizer, a lubricant and the like blended therewith, are well known as a thermal recording material by a color reaction through a fusion contact between functional units by heat. (Japanese Examined Patent Publication Nos. 43-4160, 45-14039 and the like).
Such a thermal recording material is widely used for an output sheet for a variety of printers including facsimiles, a terminal for industrial instrumentation, a terminal for medical care, a handy terminal, a POS system and a ticket vending system because recording function is beforehand effected with the support (a paper, a synthetic paper, a synthetic resin film and the like). Thus, an image can be obtained by simply heating it with a thermal head, a thermal pen, a laser beam and the like, a complicated development process is not required, and an output device having a relatively simple and compact structure, and easy maintenance can be used.
In recent years, as the application of these thermal recording materials is expanding, the environments in which they are used have are also diversifying. However, there are the following prevalent problems in their ordinary use.
Specifically, the recording layer often peels off when the thermal recording materials are brought into contact with water, the recorded images fade or discolor when the thermal recording materials are brought into contact with a plastic film or sheet comprising plasticizers, the recorded images develop color when the thermal recording materials are brought into contact with organic solvents, and the like, and thus the thermal recording materials do not sufficiently satisfy storage stability.
Therefore, as means for solving the above-mentioned problems, various methods in which a protective layer is provided on a heat-sensitive recording layer have been proposed and carried out. However, these methods require that the protective layer be crosslinked in order to highly protect the heat-sensitive recording layer under various environments. For this purpose, the protective layer is prepared by curing it with combinations of various aqueous resins and various crosslinking agents, and the like. For example, the protective layer having improved oil resistance and heat resistance (adhesion of gases to a thermal head) by using glycidyl type crosslinking agents (Japanese Unexamined Patent Application Publication No. 57-188392) has been proposed, but it has poor reactivity and water resistance.
A protective layer having an amino compound together with a glycidyl-based crosslinking agent (Japanese Unexamined Patent Application Publication No. 64-61287) has excellent water resistance and chemical resistance, but it is not suitable for labels for foods because it generates formalin. Although an aziridine-based crosslinking agent has a high reaction rate, it is unstable in an aqueous solution, which makes it difficult to handle. A method using an isocyanate-based compound (Japanese Unexamined Patent Application Publication No. 57-19036) is similarly not satisfactory. Proposals of using an epoxy-based crosslinking agent (Japanese Unexamined Patent Application Publication Nos. 49-36343, 60-68990, and 5-318926) still have problems in that a crosslinking agent having an aromatic ring is insufficient in providing resistance to plasticizers and that a crosslinking agent based on glycidyl ether of polyhydric alcohol causes a skin color development. A known polyamine-amide-epichlorohydrin modified crosslinking agent is not preferred in view of environmental conservation because it contains chlorine.
As such, the methods using crosslinking agents make it possible to solve the above-mentioned problems to some extent, but the methods are still insufficient in overcoming the problems.
Further, since these crosslinking agents are crosslinked at low temperatures, it is difficult to ensure the stability of a coating solution (variation over time) after mixing it with a resin as a main ingredient, and heat required for the crosslinking agents for sufficient crosslinking of the thermal recording material, cannot be applied thereto during the production process, in view of the properties of the thermal recording materials. For these reasons, there are problems in the process wherein after coating, the thermal recording materials must be aged over a long time period, which cause problems in its productivity.
In view of the circumstances explained above, an object of the present invention is to provide a thermal recording material, which is easy to manufacture and has excellent productivity, and also excellent in water resistance, heat resistance (running stability) and chemical resistance, by solving various problems caused by using a crosslinking agent for a protective layer.